2026 Forest River Sierra4003MB

The Sierra 4003MB has 2,891 lbs of cargo capacity between 12,609 lbs dry and its 15,500 lbs GVWR, 19% of that rating; filling its 50-gallon fresh tank at 8.3 lb a gallon uses 415 lbs of it (how to read trailer weights).

Pin weight is 2,010 lbs, 16% of the dry weight as published; it rises as the trailer is loaded and comes off the truck’s payload, not its towing rating, so check it against the payload on your own door sticker (pin weight and payload).

Off hookups: 150 gallons of grey and black capacity against 50 gallons of fresh water, so fresh water runs out first (tank sizes by length).

Against the rest of the Sierra roster this is the 4th of 9 floorplans by dry weight, in a line that runs from 10,869 lbs to 13,864 lbs. It sits in the middle of the line by weight.

Plan the tow on loaded weight, not the 12,609 lbs dry figure: half the published cargo capacity brings this plan to about 14,055 lbs, and the published GVWR of 15,500 lbs is the limit (how to read trailer weights).

Forest River’s line listing and this floorplan’s own factory page agree on all 4 shared figures. Dry weight plus cargo capacity equals the published GVWR exactly. A missing row means Forest River does not publish that figure (how figures are verified).

Forest River publishes no floorplan description for this plan in the channel this page is built from - only the numeric specification block. Rather than describe a layout from the floorplan code, which would be a guess dressed as a fact, this page carries what the factory publishes and says where it stops. 4003MB is a Sierra designation and this site does not decode it into a layout claim.

How the 4003MB differs from the other eight Sierra floorplans. Most distinctive first. Sibling pages may cover other model years than this one (2026). How these ranks are worked out.

• Pin weight, 2,010 lb: second-lightest of nine, between the 3480RL (1,965 lb) and the 3100RD (2,045 lb).
• Pin weight as a share of dry weight, 15.9%: second-smallest of nine, between the 3900HBLR (15.7%) and the 3844QUAD (16.6%).
• Dry weight, 12,609 lb: fourth-lightest of nine, between the 3990FL (12,439 lb) and the 3710HBFB (12,649 lb).
• Cargo capacity, 2,891 lb: fourth-largest of nine, between the 3710HBFB (2,851 lb) and the 3990FL (2,936 lb).
• Cargo capacity as a share of GVWR, 18.7%: fourth-largest of nine, between the 3710HBFB (18.4%) and the 3990FL (19.1%).
• Black tank, 50 gal: smallest of nine, level with the 3800RK and 3990FL; next are the 3100RD and 3480RL (52 gal).
• Grey tank, 100 gal: shared with the 3800RK and 4002FB; of nine, one smaller and five larger, nearest the 3990FL (80 gal) and the 3100RD, 3480RL and two others (104 gal).
• GVWR, 15,500 lb: shared with the 3710HBFB, 3800RK and 4002FB; of nine, three lower and two higher, nearest the 3990FL (15,375 lb) and the 3900HBLR (16,085 lb).
• Length, 42 ft 9 in: shared with the 3710HBFB, 3844QUAD and 3900HBLR; of nine, two shorter and three longer, nearest the 3480RL (38 ft 6 in) and the 4002FB (43 ft 5 in).

Next to the 3990FL, the nearest lighter floorplan by dry weight, the 4003MB is 170 lb heavier by dry weight, has a GVWR 125 lb higher, has 45 lb less cargo capacity, has a pin weight 815 lb lighter, is 9 in shorter and has a grey tank 20 gal larger; the two publish the same exterior height, fresh tank and black tank.

Next to the 3710HBFB, the nearest heavier floorplan by dry weight, the 4003MB is 40 lb lighter by dry weight, has 40 lb more cargo capacity, has a pin weight 395 lb lighter, stands 1 in lower, has a grey tank 4 gal smaller and has a black tank 54 gal smaller; the two publish the same GVWR, length and fresh tank.

Sierra's published pin weights spread further than its dry weights do, and that is the fact worth carrying into a dealership: nine floorplans run 10,319 to 13,864 lb dry - a 3,500 lb band - while pin weight runs 1,560 to 2,825 lb, nearly doubling. Two Sierras a thousand pounds apart on the scale can be far apart at the pin too. Every plan on the roster ties out exactly against its published GVWR.

A full 50-gallon fresh tank is about 415 lb of water (water counts as cargo: 49 CFR 571.120 S10.4.2), 14% of the 4003MB’s 2,891 lb cargo capacity, leaving 2,476 lb. Across 9 Sierra floorplans the water share runs 14% to 19%, and 7 give up more than this one. Nearest on that share: Sierra 3990FL at 14% and Sierra 3710HBFB at 15%. Your own unit’s figure is on the label inside its entry door (49 CFR 571.120 S10.4.3).